1. Field of the Invention
The invention concerns the manufacture of integrated circuits, and more precisely, of integrated circuits having a processor for the processing of signals, namely a circuit capable of performing data-processing operations under control by instructions.
2. Description of the Prior Art
In many integrated circuits, it is necessary to have a clock available. A signal processor, in any case, requires a clock that provides for the smooth sequencing of the processing operations performed.
The clock may be set up by means of an oscillator internal to the integrated circuit, or else the clock signals may be applied from the outside of the integrated circuit, at a terminal specially reserved for this purpose.
The use of an external clock obliges the user of the integrated circuit to provide for this clock, and this indirectly increases the cost of the clock for the user. Furthermore, this makes it necessary to have a circuit terminal reserved for the clock.
When an oscillator is made on an integrated circuit, the difficulty is that of obtaining precisely a desired frequency. In fact, manufacturing variations resulting from the technological processes used are such that it is not possible to obtain a frequency with sufficient precision. The variation in the natural frequency of oscillation for two identical oscillators that have gone through the same production line is easily 100% or even more. This results from the fact that the manufacturing processes involve steps for doping, diffusion of impurities at high temperature, deposition of thin insulator layers etc. It is not very easy to achieve mastery over the reproducibility of these steps from one circuit to the next one.
Either the oscillation frequency is not of vital importance, and then it is possible to accept having an oscillator that is entirely integrated into the circuit, without any external terminal for the reception of a clock signal. Or, on the contrary, the frequency is a vitally important parameter and, in this case, the method generally used is that of connecting, to the exterior of the integrated circuit, elements (generally resistors or capacitors) for adjusting the oscillator present at the circuit. These adjusting elements have a value that is very precisely known, because they are not subject to the same manufacturing variations or else because they have been selected. However, the drawback is that they must be connected directly to the oscillator of the integrated circuit and, consequently, they make it necessary for the integrated circuit to have additional external pins, specially reserved for this purpose. However, the external additional pins should be avoided as far as possible for they are the biggest factors of cost in integrated circuits.
In a signal processor, namely an electronic circuit capable of carrying out various signal processing tasks under control by instructions, the clock that determines the sequencing of the operations carried out by the microprocessor is a very important circuit element and its frequency has to be properly determined. In the signal processing circuits made at present, the clock is external or else it is internal and adjusted by external precise components (quartz elements, resistors, capacitors etc.).
An aim of the invention is to propose an integrated circuit comprising an internal clock that does not suffer the drawbacks just described, and notably the drawbacks resulting from the technological variations among circuits that are identical in theory.
Another aim of the invention is to enable the making of an integrated circuit having a clock, the frequency of which can be defined easily and with precision, without making it necessary to have specific connector pins to connect external adjusting elements to an internal oscillator.
Finally, an aim of the invention is, very specially, the making of a signal processor having a clock to provide for the sequencing of its own operations, said clock having a frequency that can be defined easily by the processor itself.